The identification and recording of people via biometric features is becoming increasingly important. The fingerprint and/or handprint plays an important role alongside other recordable biometric features. On the one hand, there are systems which are used to verify biometric features in which there must therefore be a match with stored features, for example in order to enable entry or access control. Other systems are used for identification by searching and storing in reference databases, for example in the case of border controls at airports or in the case of identity-checking by the police. For the latter systems there is a large number of requirements in terms of the quality, the resolution and the faithfulness to the original of the captured images of the skin textures. Not least because of the high demands of organizations entrusted with identity-checking measures, such as for example the Federal Bureau of Investigation (FBI), there is a high degree of standardization with these systems in order, on the one hand, to ensure as definite an identification as possible and, on the other hand, to make data sets which were captured by different systems comparable. For example, such systems must have a resolution of at least 500 ppi, which corresponds to a pixel pitch of 50.8 μm. In addition, particular demands are made on the contrast transfer function (CTF), the signal-to-noise ratio (SNR) and the distortion. Finally, the color scale must comprise at least 200 greyscale values and the field of view must be illuminated as homogeneously as possible both in the immediate vicinity of the pixel and in the image as a whole.
All of the demand criteria require a balanced and high-quality system design. In the case of an optical system, this means, for example, that not only the acquisition sensor or acquisition sensors must satisfy the demands but also the illumination and all of the other components necessary for the image generation.
For recording finger and handprints which fulfil the named high quality demands, at the present time, optical arrangements are predominantly used which operate according to the principle of disturbed total internal reflection. For this, a prism is applied, the surface of which provided for capturing the print must be larger than the surface required for capturing the print because of mechanical and optical demands. The size of the prism resulting from this often as a larger component in the capturing channel has a decisive influence on the minimum overall size and the minimum weight of a device. On the other hand, however, the high image quality permits a rapid and reliable recording and identification of the people, in particular in the case of applications where, in addition to forensic accuracy, a high throughput of people also plays a role, such as for example in the case of border controls. In addition to the overall size and the weight already mentioned, there are further disadvantages in the necessary use of complex mechanical components and in a technically complex and time-consuming assembly and calibration. The user guidance in the case of such devices takes place via information which is presented outside the contact surface, such as e.g. by an adjacent screen. During the capture of skin prints, the person must therefore switch their view repeatedly between the contact surface and the screen.
Miniaturized arrangements with imaging optical systems, as are described for example in U.S. Pat. No. 7,379,570 B2, do not generally satisfy the high standards which are specified for example by the FBI and still limit a further miniaturization of the devices because of the optical beam path. Ultrasonic or piezoelectric sensors, as are known for example from U.S. Pat. No. 4,394,773, and capacitive sensors, as are described for example in U.S. Pat. No. 5,325,442, can capture fingerprints non-optically. Devices based on ultrasonic sensors are not yet on the market. Capacitive sensors, in turn, to date exist only for one or two finger pictures. To date, all of the non-optical principles have the disadvantage that no information can be directly displayed on the sensor. Membrane keyboards, as are described for example in US 2005/0229380 A1, do not fulfil the necessary criteria.
In order to combine the advantage of high image quality which can be achieved with disturbed total internal reflection with small overall size, approaches were already described in US 2012/0321149 A1. The fingerprint sensor disclosed there, in which the finger is placed on a TFT display, captures a fingerprint and transmits this via an electronic system to a computer system. The brightness profile corresponding to the fingerprint forms—as in the case of arrangements with prisms—in that the epidermal ridges lying on the surface of the TFT display disturb the internal reflection of the light from the light source, while in the papillary lines, i.e. the skin valleys, no contact occurs between skin and TFT surface and there the light from the light source is reflected internally on the surface of the TFT. A negative image of the fingerprint image thus forms on the light-sensitive areas of the TFT. However, this solution assumes that the upper substrate has a minimum thickness so that the light can strike the light-sensitive areas of the TFT. In addition, it is necessary for the illumination to fulfil certain requirements with regard to the direction of incidence and aperture angle, which significantly increases the technical complexity of the illumination and also the space requirement thereof. The embodiments of illuminations described in US 2012/0321149 A1 are not suitable or are only suitable to a limited extent for large capture surfaces for more than one or two fingers since they are associated with great complexity.
A further concept for a flat construction without an imaging optical system is described in U.S. Pat. No. 7,366,331 B2. Here, the light is coupled into the finger laterally by means of a flat illumination and scattered in the adjacent part of the skin. The finger is in contact with a transparent layer between finger and sensor. Thus light preferably couples out of the skin peaks into this layer and can then be detected by the flat sensor. This concept requires illumination wavelengths within the transparency range of the finger, i.e. in particular in the near and mid-infrared range, and thus involves considerable ambient light problems. Although the recommended use of infrared filters and infrared illumination reduces these problems, the sensitivity of usual sensors is then lower and the absorption of the finger is higher, which worsens the signal-to-noise ratio. Optionally used narrow-band spectral filters must be adapted in accordance with the wavelength of the illumination and generate additional technical complexity. The lateral illumination causes problems for the homogeneity of the illumination, in particular it prevents the simultaneous capture of several fingers, since these would shadow each other. This concept is therefore only suitable for capturing one finger. In addition, a light shield is necessary in order to prevent portions of the illumination arriving directly in the sensor. The lateral illumination and the light shield increase the size of the device and it becomes more complex, less flexible and more prone to errors.
In EP 2 711 869 A2, an arrangement and a method for capturing fingerprints are described. The user who places a finger or a hand on a capture surface obtains information by means of pictograms, images or the like on a display unit which is close but spatially separate from the capture surface. This information reveals, for example, whether, for example, the placement position and/or the contact pressure are acceptable or, for example, whether all of the fingers are lying on or whether some fingers are missing, if the whole hand is to be captured. Because of the spatial separation of the capture surface and display it is cumbersome for the person whose prints are to be taken to correct the position of the autopodia on the capture surface, since the transmission of the image represented on the display surface does not take place immediately and intuitively on the action or a required movement for example of individual fingers. Furthermore, because of the spatial separation of the capture surface and the display unit, there is the risk that the user does not place his fingers on the contact surface but on the display. That takes place in particular when the representation on the display shows the fingers to be captured. Capturing rolled fingers requires particularly great attention during the capturing process since a satisfactory fingerprint picture is only possible with a continuous speed and continuous pressure. This is usually achieved in that the capturing process is represented on a display in real time. In the case of the spatial separation of the capture surface and display it is very difficult for the user to synchronize the capturing process and the display. In practice, this leads too often to flawed fingerprint pictures which must be repeated.